Layered silicate minerals

McDowell, S.D. and Elders, W.A. (1980) Authigenic layer silicate minerals in borehole Elmore 1. Salton Sea geothermal field, California, U.S.A. Contr. Mineral. Petrol., 74, 293-310. 

Layer silicate minerals have a high selectivity of trace transition and heavy metals and greater irreversibility of their adsorption. Some chemisorbing sites such as -SiOH or AlOH groups may be at clay edges and form hydroxyl polymers at the mineral surface. Another possible reason for the high selectivity may be hydrolysis of the metal and strong adsorption of the hydrolysis ion species. 

Most serpentines and other layered silicate minerals, such as micas and clays, are composed of tetrahedral and octahedral sheets that lie virtually flat. In chrysotile samples, however, the layers curl, rolling up like a carpet, to form concentric hollow cylinders (Fig. 2.4). The average diameter of a cylinder, which is a chrysotile fibril, is about 25 nanometers (25 nm = 0.025 mi-... 

Currently, the only biological barrier registered as a biochemical pest control agent is kaolin, a clay mineral [47]. Kaolin is a ubiquitous clay substance found in soils worldwide and consists of a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra [Al2Si205(0H)4]. 

Roy, D.M. and Roy, R., 1954. An experimental study of the formation and properties of synthetic serpentines and related layer silicate minerals. Am. Mineralogist, 39 957-975. 

STRUCTURAL COMPARISON OF SEVERAL 2 1 LAYER SILICATE MINERALS... 

TABLE 3.1 Chemical Composition and Charge Characteristics of Representative Layer-Silicate Minerals... 

There is not much resistance to weathering in these minerals because of the relative lack of Si—O—Si bonding, especially in island silicates such as olivine. Layer silicate minerals rich in Mg (e.g., trioctahedral smectites, chlorite, serpentine) may form from the siliceous residue if leaching does not deplete in the weathering zone. 

Harder, H. 1978. Synthesis of iron layer silicate minerals under natural conditions. Clays and Clay Minerals 26 65-72. 

This list may be lengthened when the effects of adsorption are considered, because a molecule bound on a surface may be activated for photodegradation even if the reaction is not favorable for the same molecule dissolved in solution. For example, paraquat apparently photodecomposes more rapidly when adsorbed on layer silicate minerals than when in solution (Helling et al., 1971). This phenomenon may be related to the fact that adsorption on clay shifts the UV absorption band of paraquat to longer wavelength (256 - 275 nm) and closer to the atmospheric window for UV light (see Figure 10.20). In other words, adsorption on clay increases the probability that paraquat will absorb UV radiation and thereby decompose. 

Layered aluminosilicates are the most important secondary minerals in the clay fraction of soils. When layer silicate minerals are clay or colloidal size (<2 gm effective diameter), their large surface area greatly influences soil properties. Most of the important clay minerals have similar silicate structures. Inasmuch as clay minerals are such important clay components, and as different clay minerals can change sail properties greatly, an understanding of soil properties begins with an understanding of silicate structures. 

A crystal is an arrangement of ions or atoms that is repeated at regular intervals in three dimensions. The smallest repeating three-dimensional array of a crystal is called the unit cell. The unit cell dimensions a and b (the x and y dimensions) are constant for a given mineral the c (or z) dimension is also constant, except for the special case of swelling layer silicates (Fig. 5.3). The basal plane is the a-b plane. The chemical composition of layer silicate minerals is normally expressed as one-half of a unit cell, to simplify the chemical formulas. 

Layer silicates, sheet-like phyllosilicates such as the familiar micas, are in primary rocks and in soils. The soil minerals are often called clay minerals. Since other components can also be in the clay fraction, layer silicates is a mom accurate term. A typical layer silicate is a combination of a layer of Al-, Mg-, or Fe(II)-0 octahedra plus one or two layers of Si-0 tetrahedra. The tetrahedral and octahedral sheets bond together by sharing oxygens at the corners of the tetrahedra and octahedra. Layer silicate minerals are differentiated by (1) the number and sequence of tetrahedral and octahedral sheets, (2) the layer charge per unit cell, (3) the type of interlayer bond... 

The 2 1 layer silicate minerals are sometimes defined on the basis of the number of octahedral positions occupied by cations. When two-thirds of the octahedral positions are occupied, such as in pyrophyllite (Al2Si40io(OH)2), the mineral is call dioctahedral, when all three positions are occupied, such as in talc (MgjSL OioCOH ), the mineral is called trioctahedral. This difference in composition of layer silicates can be fairly easily determined by x-ray diffraction, because each substitution slightly changes the dimensions of the unit cell. 

Isomorphic substitution occurs during crystallization of layer silicate minerals in magmas and in soils. If the primary cation is unavailable as the unii cell forms, another cation can sometimes squeeze in. The resulting permanent charge is essentially independent of the soil solution composition surrounding the particle. Isomorphic substitution is the principal source of negative charge for the 2 1 and 2 1.T layer silicates, but is of minor importance for the T.l minerals. 

An impressive property of colloids, including layer silicate minerals, is their large area of reactive surface. Various physical and chemical properties, including water retention and cation exchange capacity, are highly correlated with the surface area of soils. Several techniques estimate the amounts of reactive surface area of soils and are briefly described below. 

Most of the colloidal properties of SOM are due to humus. Humus is highly colloidal and is x-ray amorphous rather than crystalline. The surface area and adsorptive capacities of humus per unit mass are greater than those of the layer silicate minerals. The specific surface of well-developed humus may be as high as 900 x 103 m2 kg-1 its exchange capacity ranges from 1500 to 3000 mmol(+) kg-1. 

The sum of exchangeable Ca2+, Mg2+, K+, Na+, and Al3+ generally equals, for practical puiposes, the soil s cation exchange capacity (CEC). The CEC varies from 10 mmol(+) kg-1 for coarse-textured soils to 500 to 600 mmol(-f) kg-1 for line-textured soils containing large amounts of 2 1 layer silicate minerals and organic matter. 

Another important facet of nitrogen availability in acid soils is the pH dependence of ammonium-ion fixation between the lattices of expanding layer-silicate minerals. Such fixation generally decreases with increasing soil pH. Although the mechanism for this pH effect is incompletely understood, the decrease may be due to islands of hydroxy aluminium and hydroxy iron polymers, which prevent the complete collapse of mineral lattices and hence decrease NH4 fixation. 

Permanent charge Permanent charge on solid surfaces results from the isomorphous substitution in 2 1 clay minerals, for example, in layered silicate minerals, replacement of either the Si or the AF+ cations with cations of lower charge can increase negative charges on solid phase. Isomorphic substitution occurs during mineral formation and is largely unaffected by environmental conditions. 

Kaolinite (AI Si O fGH) ) is a layered silicate mineral. The structure contains one tetrahedral sheet linked to one octahedral sheet of alumina octa-hedra through oxygen bonding. It possesses low shrink-swell capacity and a low cation exchange capacity of 1-15 meq/100 g. 

---